In conventional processing methods, photographic products comprising silver halide grains carrying a latent image formed by exposure to radiation are developed by immersing the photographic product in an alkaline solution containing a reducing developing agent. The developing agent is generally an organic compound selected from the di- and polyhydroxybenzenes, aminophenols and reductones. The dihydroxybenzenes that are used most often are for example hydroquinone and its derivatives, and catechol and its derivatives. Examples of aminophenols include 4-amino-1-hydroxybenzene and its derivatives. The most important reductones include ascorbic acid, D-isoascorbic acid and their derivatives and salts. These organic developers can be used along with an auxiliary developing agent such as phenidones or Elon.RTM..
It is also known that inorganic developing compositions can be used which contain, as a developing agent, metallic ions that are capable of changing valency in order to be able to reduce the silver ions to metallic silver.
The activity of these inorganic developers can be improved by the presence of a complexing agent able to form an organometallic complex with the metallic ion of higher valency. By thus eliminating the oxidized form of the metallic ion as it is formed, the thermodynamic force of the reduction reaction of the silver and the corresponding oxidation of the metallic ions is maintained. According to T. H. James, Photo. Sci.&Eng., 4(5) 271-280, (1960) these organometallic complexes can be used to develop different types of silver halides within a wide pH range. The mechanism of such a development was described by H. J. Price in the J. Photo. Sci.Eng., 14(6), 391-396, (1970) and 19(5), 283-287, (1975).
French Patent 1,068,805 describes the use of complexes of iron (II) and titanium (III) with particular aliphatic aminopolycarboxylic acids, and the salts of these acids, in order to develop silver halide photographic products, after exposure. The described aminopolycarboxylic acids are characterized in that the amino group or groups comprise at most one hydrogen atom bonded directly to the nitrogen atom. The aminopolycarboxylic acids cited are ethylenediamine tetraacetic acid (EDTA), methylenediamine tetraacetic acid (MDTA), nitrilotriacetic acid (NTA) and diethylenetriamino diacetic acid.
U.S. Pat. No. 3,887,375 describes an inorganic developer consisting of chelated ferrous ions and ascorbic acid. According to this patent, the ascorbic acid provides air stability of a developing composition containing an inorganic developer.
If the conventional organic developers, for example developers containing hydroquinone derivatives, are compared with so-called "inorganic" developers comprising organometallic complexes, it appears that organic developers may have certain advantages. They may, for example, be more readily soluble in water. They can also be used within a wider pH range, which makes it possible to use less alkaline processing baths. In addition, the organometallic complexes form reversible redox systems, which makes it possible to easily determine and control the redox potential of the developing bath and to regenerate the oxidized form of the metallic complex into its initial reducing form simply by reduction of the processing bath.
The possibility of regenerating this type of developing bath makes it possible to obtain ecological processing by minimizing the volume of effluents.
As is described by the art, metallic developers are always associated with an organic complexing agent in order to form an organometallic complex. This complexing agent is generally EDTA, which is particularly effective but has low biodegradability.
Furthermore, these inorganic developers exhibit low stability in air, linked to the oxidation of the developing composition. G. M. Haist et al, in Photographic Engineering, 7(3), 182 (1956), indicate that the practical application of these inorganic developers is limited because it requires oxygen-free atmospheres. In U.S. Pat. Nos. 2,453,323 and 3,567,441, it is asserted that the development must be carried out in a non-oxidizing atmosphere, for example under nitrogen.
It would be desirable to have an inorganic developer composition that is more ecological and has better resistance to oxidation in air.